1. Field of the Invention
The present invention relates to a method for casting a hollow camshaft for an internal combustion engine and, more particularly, to a method for casting a camshaft having a hollow portion having a variable inner diameter.
2. Description of Related Art
In general, a camshaft formed as a hollow body defining an elongated inner cavity is produced by a casting process. To produce the camshaft, an elongated core is positioned within an iron-casting mold.
The core is supported at one or more locations along its length and spaced a predetermined distance from an inner surface mold. However, in case of using a sand core, the sand core has been found to fail entirely beyond a given length in actual practice.
Thus, a scheme to solve the problem is disclosed in U.S. Pat No. 4,832,107. This patent discloses a glass material which is used as a molding core to have an advantage that braces are not needed. However, the casting process using a glass material as a molding core has a problem that the glass material should be removed after completing the casting, thereby wasting time and effort to produce a product. The process further has a problem that the inner surface of the casting becomes rough since gas can not be exhausted during the casting process.
Accordingly, a method using a sand core has been again improved such that a cavity is formed in the sand core along its length to improve a ventilation characteristic during the casting process.
However, when the sand core having the cavity is used, a rod is inserted into the cavity of the sand core such that both ends of the rod project from both ends of the sand core, respectively. The projecting portions of the rod are utilized as a core print whereby the camshaft produced by the core can not help becoming a hollow camshaft having both ends communicating with each other.
The hollow camshaft as described above in actual practice can not be used for a variable valve timing VVT type of engine which varies open and close timings of intake and exhaust valves in accordance a with vehicle's speed because of a problem which will be described below.
That is, to vary the open and close timings of the intake and exhaust valves according to the vehicle's speed, means for varying the open and close timings should be mounted on the camshaft. In this case, when the means, for example, a pulley, is fixed on the camshaft, a front end of a set screw undesirably penetrates the hollow portion of the shaft.
Accordingly, there have been improvements in methods for mounting the means on the hollow camshaft. As one method, a hollow cam shaft having a hollow portion having a substantially reduced inner diameter for coupling the set screw for a pulley is provided.
However, providing such a camshaft is impossible with the conventional casting method.